1. Field of the Invention
The present invention relates to a powder coating composition used to coat a molded product having wrinkles and cavities in its surface, such as a die-cast product, an extrudate or an injection-molded product made from a zinc alloy, an aluminum alloy, a magnesium alloy and the like, and particularly, to a powder coating composition which is suited to be applied directly onto a to-be-coated-surface without need for a pretreatment such as the sanding of the surface and the filling of the surface by a patty, thereby hiding a molding failure of the surface satisfactorily to form a coated surface excellent in design, and which does not require the use of an organic solvent or the like, whereby an air pollution is prevented, and the danger of a fire is reduced remarkably.
2. Description of the Related Art
Molded products made from various alloys such as zinc, aluminum and magnesium alloys, other metals and various plastic materials, are usually formed by a die-casting, an extrusion, an injection molding and the like. In such molded products, fine grooves called wrinkles, small bores called cavities and the like are created in their surfaces, thereby causing the coating defects due to these fine grooves and small bores are caused.
It is a conventional practice to coat such a molded product through many steps, such as steps of prime-coating and face-coating of the surface of the molded product which is to be coated, using a solvent-type coating material and the like, after the pretreatment of the surface such as the sanding, the filling by a patty and the like.
For this reason, the above-described coating process suffers from a problem that not only the coating steps are complicated, but also the use of the solvent-type coating material causes the air pollution, the dangers of the generation of a fire and the like.
Accordingly, it is an object of the present invention to provide a powder coating composition, which is coated directly onto the surface of a molded product having wrinkles, cavities and the like in its surface without the pretreatment such as the sanding of a surface to be coated, the filling of the surface by a patty and the like, whereby the coating process is simplified, and a molding failure of the surface is hidden satisfactorily, thereby forming a coated surface having an excellent design property, and which does not use a solvent such as an organic solvent, whereby the air pollution due to the solvent is prevented and the danger of a fire is reduced remarkably, thereby overcoming the drawbacks associated with the prior art.
To achieve the above object, according to the present invention, there is provided a powder coating composition for use in the coating of a molded product having wrinkles, cavities and the like in its surface, comprising a single film forming resin or a plurality of film forming resins having a melt viscosity of 60 Paxc2x7s/165xc2x0 C. or less and selected from the group consisting of an epoxy resin, a polyester resin and an acrylic resin, and at least two fillers having different particle size in a range of 3 to 100 xcexcm.
When the powder coating composition according to the present invention is applied onto a molded product having wrinkles and/or cavities in its surface and formed by a die-casting process, an extruding process, an injection-molding process and the like from any of various alloys such as a zinc alloy, an aluminum alloy, a magnesium alloy and the like, other types of metals, and various plastic materials, the following effects are provided:
(1) A pretreatment such as sanding, a patty-filling of a surface to be coated is not required, and the powder coating composition can be applied directly to the surface to be coated, leading to simplified coating steps;
(2) The molding failure of the surface to be coated can be hidden satisfactorily;
(3) It is possible to form a coated surface which is excellent in design property (realization of a pattern); and
(4) A solvent such as an organic solvent and the like is not used and hence, the air pollution by a solvent is prevented, and the danger of a fire is reduced remarkably.
The present invention will now be described in detail.
A powder coating composition according to the present invention basically contains a film forming resin and a filler.
Examples of the film forming resin is an epoxy resin, a polyester resin, an acrylic resin and the like. These resins are used alone or in combination of two or more of them. Fine grooves due to wrinkles, small bores due to cavities and the like formed in the surface of a molded product are hidden by using the film forming resin.
The filler used in the present invention comprises an organic or inorganic powder having an average particle size in a range of 3 to 100 xcexcm. According to the present invention, at least two types of such fillers having different particle sizes are used in combination.
Examples of organic fillers are nylon, polyacrylonitrile, polymethylmethacrylate (PMMA), benzo-guanamine resin, a polyester resin, a silicone resin, polytetrafluoroethylene (PTFE), polystyrene, polyethylene, a phenol resin, a polyurethane resin, collagen, and the like. Examples of inorganic fillers are ceramics, shirasu, glass, settleable barium, calcium carbonate, alumina, quartz, clay, mica, metal powder and the like.
Examples of the combination of at least two fillers are the combination of organic fillers, the combination of organic and inorganic fillers and the combination of inorganic fillers. It is preferable that one of the two fillers has an average particle size in a range of 5 to 50 xcexcm.
Examples of preferred combinations of fillers are the combination of a glass filler having an average particle size of 18 xcexcm and a polyacrylonitrile having an average particle size of 20 xcexcm, the combination of a feldspar powder having an average particle size of 4.5 xcexcm and a polyester filler having an average particle size of 20 xcexcm, the combination of a feldspar filler having an average particle size of 4.5 xcexcm and a polyacrylonitrile filler having an average particle size of 15 xcexcm, the combination of a polyacrylonitrile filler having an average particle size of about 20 xcexcm and a polyester filler having an average particle size of 20 xcexcm, the combination of a glass filler having an average particle size of 18 xcexcm and a ceramic filler having an average particle size of 45 xcexcm, and the like.
According to the present invention, the use of the two types of fillers ensures that the blister of a coating film due to cavities is overcome; wrinkles are hidden, and the uniformity of the appearance of the coating film is enhanced.
The composition according to the present invention may contains, in addition to the above-described film forming resin(s) and the fillers, any of a pigment, a curing agent, a curing promoter, a flow regulator and other various conventionally used additives in any suitable amount.
The fillers may be blended with the film forming resin in the composition according to the present invention, so that the amount of the fillers is in a range of 5 to 200 parts by weight per 100 parts by weight of the film forming resin. If the amount of the fillers exceeds 200 parts by weight, problems of a leveling failure and of generation of cracks are arisen. If the amount of the fillers is smaller than 5 parts by weight, an effect of preventing the blister of a coating film is insufficient.
According to the present invention, the film forming resin, the fillers and anyother additives are uniformly mixed together in required amounts, for example, in a super-mixer. The mixture is placed into an extruder, where it is heated into a molten state at a temperature in a range of 120xc2x0 C. to 200xc2x0 C. and kneaded. Then, the resulting mixture is extruded from the extruder, and cooled and pulverized into an average particle size in a range of 5 to 60 xcexcm by an atomizer or the like, thereby producing a powder coating material according to the present invention.
All the two or more types of fillers according to the present invention may be mixed with other constituents and then charged into the extruder. Alternatively, only at least one of the fillers may be mixed with the other constituents, and the mixture may be kneaded and pulverized in the extruder to provide a powder coating material. Then, the remaining filler(s) may be mixed with the powder coating material. In order to enhance the pattern realization and the blister preventing effect, the latter process having the first step of mixing at least one of the fillers to provide the powder coating material is preferred.